The present invention relates to the determination of cardiac output in a living being and, more particularly, to a method and apparatus for continuously determining noninvasively the cardiac output of an individual as a function of other heart and blood flow parameters.
One of the most important indicators of the status of the cardiovascular system of a patient is the cardiac output (the volume of blood pumped by the heart during a given period of time). Currently, information on this parameter is obtained reliably by invasive devices such as Swan-Ganz catheters. Because of the high cost and risks associated with these catheters, as well as the discomfort and trauma of invasive devices, they are useful only in seriously ill patients or patients with conditions that have potential for serious complications.
There is a growing need to obtain the same or equivalent information by noninvasive means. It would be particularly helpful if this information could be obtained continuously on a real time basis or by repetitive short measuring periods at predetermined intervals such as once a month or once a week.
Various noninvasive techniques have been proposed starting with Nyboer's pioneering work with a plethysmograph to determine stroke volume (amount of blood pumped by the heart during each contraction) from cardiovascular impedance change during a systolic downstroke (see Nyboer, J., Electrical Impedance Plethysmography, 1954, Charles C. Thomas, Springfield, IL, Publisher). Such techniques are disclosed, for example, in U.S. Pat. Nos. 4,450,527 of Sramek; No. Re 30,101 of Kubicek et al.; and No. 3,996,925 of Djordjevich. Both Sramek and Kubicek et al. measure impedance change in the thoracic region by means of a plethysmograph. Djordjevich uses a plethysmograph either on the trunk or on a limb.
In U.S. Pat. No. 4,437,469 of Djordjevich et al., plethysmographic apparatus for measuring thoracic impedance is combined with apparatus for measuring instantaneous blood pressure in a limb to determine, through mathematical manipulation of simultaneous values of these two measurements, stroke volume and cardiac output, as well as other hemodynamic characteristics. The outputs from the plethysmograph and the blood pressure sensor are delivered to a processor having electronic processing units capable of modifying the impedance or blood pressure signals by addition, subtraction, multiplication, division, differentiation, integration, or exponentiation. The processor performs these manipulations to solve a set of simultaneous equations based on a mechanical and electrical model of a section of a living body.
The method of Djordjevich et al. using a combination of electrical impedance and blood pressure measurements, as well as the other above-mentioned methods using electrical impedance measurements alone, are based on mathematical models of a complex physiological system. These models necessarily incorporate many simplifying assumptions. Depending upon the divergence of an actual living body from the assumed conditions of the model, the accuracy of these methods can be degraded significantly. Thus, these prior noninvasive methods require great skill in application, and there still remains a need for a noninvasive technique that simply and easily determines cardiac output.